Why did Kitty Pryde go back in time in the comics? Why not Xavier or Magneto then?

My only point is this. Some things would have to be changed from the comics no matter who was sent back in time. Therefore, they're only limited by the imagination of the writers and a consideration of what would make the most interesting story. I fully understand that Ellen Page is probably not the person they'd select to star this film. However, the reason for not choosing her has nothing to do with the rules of time travel because those rules are just as changeable as the person to send back in time.